An ultrasonic diagnostic apparatus is used to make a noninvasive checkup on a subject by irradiating him or her with an ultrasonic wave and analyzing the information contained in its echo signal. For example, a conventional ultrasonic diagnostic apparatus that has been used extensively converts the intensity of an echo signal into its associated pixel luminance, thereby presenting the subject's structure as a tomographic image. In this manner, the internal structure of the subject can be known.
Meanwhile, some people are attempting recently to track the motion of a subject's tissue more precisely and evaluate the strain and the elasticity, viscosity or any other physical (attribute) property of the tissue mainly by analyzing the phase of the echo signal.
Patent Document No. 1 discloses a method for tracking a subject's tissue highly precisely by calculating the magnitude of instantaneous displacement of a local region of the subject based on the phase difference of an ultrasonic echo signal to be transmitted and received at regular intervals and by summing the magnitudes of displacements together.
Patent Document No. 2 further develops the method of Patent Document No. 1 into a method of calculating the elasticity of a subject's tissue (e.g., an arterial vascular wall, in particular). According to this method, first, an ultrasonic wave is transmitted from a probe 101 toward a vascular 411 as shown in FIG. 8(a). And the echo signals, reflected from measuring points A and B on the vascular wall, are analyzed by the method of Patent Document No. 1, thereby tracking the motions of the measuring points A and B. FIG. 8(b) shows the tracking waveforms TA and TB showing the locations of the measuring points A and B along with an electrocardiographic complex ECG.
As shown in FIG. 8(b), the tracking waveforms TA and TB have the same periodicity as the electrocardiographic complex ECG, which shows that the artery dilates and shrinks in sync with the cardiac cycle of the heart. More specifically, when the electrocardiographic complex ECG has outstanding peaks called “R waves”, the heart starts to shrink, thus pouring blood flow into the artery. As a result, the vascular wall is dilated rapidly. That is why soon after the R wave has appeared on the electrocardiographic complex ECG, the tracking waveforms TA and TB rise steeply and the artery dilates rapidly. After that, however, as the heart dilates slowly, the tracking waveforms TA and TB gradually fall to their original levels and the artery shrinks gently. The artery repeats such a motion cyclically.
The difference between the tracking waveforms TA and TB is represented as a waveform W showing a variation in thickness between the measuring points A and B. Supposing the variation of the thickness variation waveform is ΔW and the reference thickness between the measuring points A and B during initialization is Ws, the magnitude of strain ε between the measuring points A and B is calculated by:ε=ΔW/Ws 
The elasticity Er between the measuring points A and B is given by:Er=ΔP/ε=ΔP·Ws/ΔW where ΔP is the blood pressure difference at this time.
Therefore, by measuring the elasticity Er for multiple spots on a tomographic image, an image representing the distribution of elasticities can be obtained. If an atheroma 412 has been created in the vascular wall as shown in FIG. 8(a), the atheroma 412 and its surrounding vascular wall tissue have different elasticities. That is why if an image representing the distribution of elasticities is obtained, it can be determined whether an atheroma has been created or not or where the atheroma is located.
Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 10-5226
Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2000-229078